Driver side rear brake and signal lights are not working

The external lighting on this van is controlled by the smart junction box. If there is an issue with the circuit or bulb, the FET transistor inside the smart junction box will open and not allow voltage out to the bulb. Most of the times a quick reset of the module by unhooking the battery will reset the module, as long as there is no concern still present. Add on lighting to these circuits are known to cause the module to fail completely as well. Here is a diagram that I illustrated to help:

where would one buy a replacement module and where is it located in the vehicle?

The smart junction box is located under the dash on the driver side (see picture). I don't know if the aftermarket world carries them because I am at the dealership level, so Ford can get you one for sure. That is if in fact it does need one! I would try to either unplug the module for three minutes, or the battery for three minutes. This module also must be programmed if replaced. Retrieving trouble codes from the module and performing pinpoint tests should be done before replacement.